[Resolved] Unable to Migrate User to O365 due to "Target user 'XYZ' already has a primary mailbox"

I recently was working on an issue where an OnPremises Exchange Mailbox couldn´t be moved into Exchange Online (aka Office 365) via New-MoveRequest. The error message was "Target user 'XYZ' already has a primary mailbox". The solution to fix that was quite easy

The root cause here was that somebody enabled the Exchange Online feature and then created the Exchange OnPremises mailbox. This caused that the user had two mailboxes. One in the cloud, the other one in Exchange Online.

Solution:

1.) As this user never used any office 365 features and didn´t got any emails in the cloud I decided to fully delete him in the cloud via: